1. Field of the Invention
Present invention relates to a process for producing hexachlorodisilane (Si2Cl6), having a high purity, which can be recovered efficiently from the exhaust gas discharged from a process for producing polycrystalline silicon. Moreover, the present invention also relates to a process recovering tetrachlorodisilane (Si2H2Cl4) together with hexachlorodisilane.
2. Discussion of the Background
Hexachlorodisilane (Si2Cl6) is useful as a raw material in the production of amorphous silicon thin films, in base glass for optical fibers, and for disilanes. The conventional method for producing hexachlorodisilane is by chlorinating an alloy powder containing silicon to form a mixed gas of a poly-chlorosilane. The mixed gas is then cooled, condensed, and distilled to separate hexachlorodisilane. (Japanese Patent Laid Open No. Syou 59-195519). In addition, hexachlorodisilane has also been produced by a process in which a ferrosilicon is reacted with chlorine gas using a stirring and mixing horizontal-type reaction tube. (Japanese Patent Laid Open No. Syou 60-14590). However, these processes use a low-grade metallic silicon as a raw material which leads to the unavoidable contamination of the hexachlorodisilane produced. Specifically, metallic impurities in low-grade silicon form contaminates in the product mixture that are difficult to separate from the desired hexachlorodisilane. This is especially true when the silicon contains titanium and aluminum impurities, since the resulting chlorides of titanium and aluminum (i.e., TiCl4 and AlCl3) have a boiling point that is very close to the boiling point of hexachlorodisilane. The presence of these by-products therefore usually make the recovery, by distillation, of high purity hexachlorodisilane very difficult.